Organic el luminescene device

ABSTRACT

An organic EL light-emitting device includes a substrate, thin films having a reflecting function formed on the substrate, an organic EL light-emitting layer, and upper electrodes. The thin films having a reflecting function are formed from an amorphous alloy, whereby there can be provided an organic EL light-emitting device having reflective films that have all of a reflecting function, a function of shielding transistors from light, and an electrode function, and moreover have little surface unevenness.

TECHNICAL FIELD

The present invention relates to the structure of an organic EL(electroluminescent) light-emitting device and a method of manufacturingthe same.

BACKGROUND ART

Generally, with an organic EL light-emitting device, transparentelectrodes are provided on a glass substrate, an organic ELlight-emitting layer is provided thereon, and layers having thefunctions of both electrodes and reflective films are further formed ata rear surface using aluminum, silver or the like for increasing theamount of light extracted to the outside; light is then extracted outfrom the glass surface.

On the other hand, there have been trials into using polycrystallinesilicon TFTs as an effective driving method in the case of applying alight-emitting device to a display or the like. With this TFT drivingmethod, as conventionally there is again a method in which light isextracted from the TFT substrate side, but in this case the transistorsmust be placed in gaps between light-emitting parts, and hence there areproblems such as the area of the TFT devices being restricted.

One can thus envisage a method in which the transistors are made so asto extend out as far as the regions of the light-emitting parts, and thelight is extracted from the opposite side to the substrate (a topemission method). When this method is adopted, it is necessary to formon the TFT substrate reflective films that have both a reflectingfunction and a function of shielding the transistors from light.Furthermore, these reflective films preferably also function aselectrodes.

When forming an organic EL light-emitting layer on these reflectivefilms, unevenness of the surface of the reflective films becomes aproblem. The organic EL light-emitting layer is thin, having an overallthickness of approximately 200 nm, and moreover out of the organic ELlight-emitting layer, an electron transport layer where electric fieldconcentration occurs is extremely thin at approximately 30 nm. There isthus a problem that if there is severe unevenness on the surface onwhich the device is formed, then electric field concentration willoccur, short-circuiting of the device will occur, and parts where lightcannot be emitted (dark spots) will be formed.

When manufacturing a high-quality top emission type organic EL device,it is thus important to form reflective films that have all of areflecting function, a function of shielding the transistors from light,and an electrode function, and moreover have little surface unevenness.

Moreover, an extremely thin organic EL light-emitting layer as describedabove is also used in organic EL light-emitting devices in which notactive matrix driving using TFTs but rather passive matrix driving iscarried out. In the case of carrying out passive matrix driving, it isthus again important to form reflective films having little surfaceunevenness.

SUMMARY OF THE INVENTION

To solve the above-described problems, the present invention is directedto a structure in which the reflective films are formed from an alloyhaving a specific element ratio giving an amorphous phase, wherebyreflective films having little surface unevenness are obtained.

An organic EL light-emitting device that is a first embodiment of thepresent invention is an organic light-emitting device in which thinfilms having a reflecting function are formed in advance on a surface onwhich the device is to be formed, and then light-emitting parts areformed thereon, and is characterized in that the thin films having areflecting function are formed from an amorphous alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to certain preferredembodiments thereof, wherein:

FIG. 1 is a schematic sectional view of anorganic EL light-emittingdevice of the present invention;

FIG. 2 is a graph comparing the current-voltage characteristic fororganic EL light-emitting devices of Examples 1 to 3; and

FIG. 3 is a histogram of the reverse bias breakdown strength for theorganic EL light-emitting devices of Examples 1 to 3;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The following is a detailed description of an organic EL light-emittingdevice of the present invention. The organic EL light-emitting device ofthe present invention is shown in FIG. 1. The organic EL light-emittingdevice comprises a substrate 1, thin films 2 having a reflectingfunction formed on the substrate, an organic EL light-emitting layer 3,and upper electrodes 4.

The results of measurements using an AFM on a surface region (2 μmsquare) in the case that 3 nm or 100 nm of Al, which is generally usedfor reflective films, was deposited by vapor deposition on a glasssubstrate are shown in Table 1. From Table 1, it can be seen that thesurface unevenness increases as the Al grows. TABLE 1 Surface roughnessof Al thin films Average surface roughness (nm) Peak to valley (nm)Glass 0.21 3.06 Al (3 nm)/glass 088 11.74 Al (100 nm)/glass 2.74 38.85

The reason that the unevenness increases as the film grows in this wayis thought to be that Al readily crystallizes. That is, Al that hasreached the substrate surface can migrate relatively freely over acertain region of the surface (surface migration). In this process, ifthere are places where the adsorption potential is large, then theresidence time there will be long, and as a result the film growth rateat these parts will be high, causing unevenness to be produced. In thecase of a simple metal, these parts where the adsorption potential islarge are parts where the crystallinity is high.

Upon actually taking sectional TEM images of Al thin films, it has beenfound that the film thickness is lower in twin crystal parts (partsbetween two crystals) where the crystallinity is poor. It is thoughtthat if such unevenness in the residence time over the surface could beeliminated, then a flat metal surface would be obtained.

One can envisage two methods for doing this. One method is to grow thereflective films 2 as a perfect single crystal. However, to grow thereflective films 2 as a perfect single crystal, generally epitaxialgrowth must be carried out, and hence this is not practical whenconsidering application to an organic EL light-emitting device in whichthe device is formed on glass or an organic film. The second method isto grow the reflective films 2 as a completely amorphous phase. In thecase of growing the reflective films 2 as an amorphous phase, partshaving a large adsorption potential will not arise, and hence it will bepossible to form flat films.

As a material that can be grown as an amorphous phase, it is practicalto use an alloy. For the alloy to form an amorphous phase, it ispreferable for the mixing enthalpy of the elements constituting thealloy to be negative, and for the atomic radius ratio r/R (where R>r) ofthe constituent elements to be not more than 0.9, preferably not morethan 0.85. As such a combination, 1) a transition metal-phosphorusalloy, 2) a transition metal-boron alloy, or 3) a transitionmetal-lanthanide alloy can be used. Note that in the presentspecification, ‘transition metal’ means an element from groups 3 to 12of the periodic table excluding the lanthanides and the actinides series(e.g. in the case of group 4 of the periodic table, the elements from Scto Zn). Moreover, in the present specification, ‘lanthanide’ means anelement having an atomic number from 57 (La) to 71 (Lu).

In the case of using a transition metal-phosphorus alloy for thereflective films 2, the alloy may contain 10 to 50 at %, preferably 12to 30 at %, of phosphorus. In the case of using a transition metal-boronalloy for the reflective films 2, the alloy may contain 10 to 50 at %,preferably 12 to 30 at %, of boron. Alternatively, in the case of usinga transition metal-lanthanide alloy for the reflective films 2, thealloy may contain 10 to 50 at %, preferably 25 to 50 at %, of alanthanide.

Moreover, one element may be used as the transition metal, or two ormore elements may be used. In the present invention, preferabletransition metals include Ni, Cr, Pt, Ir, Rh, Pd and Ru, with Ni and Crbeing particularly preferable.

In the present invention, the higher the transition metal content, thehigher the reflectivity of the reflective films 2 can be made. Theoptimum transition metal content depends on other desired properties,and can be easily determined by a person skilled in the art.

In the present invention, the reflective films 2 can be formed on thesubstrate using a method such as vapor deposition or sputtering. Thesubstrate used here may be a TFT substrate on which TFTs for drivinghave already been formed. Moreover, in the case of forming a device withpassive matrix driving, a glass substrate, a plastic substrate or thelike can be used.

In the present invention, the reflective films 2 have a thickness of atleast 20 nm, preferably 70 to 150 nm. Through having such a thickness,good reflectivity, and good ability to shield the TFTs from light (inthe case of using TFTs) can be realized.

Moreover, in the present invention, due to being made of an electricallyconductive alloy, the reflective films 2 can also be used as lowerelectrodes of the organic EL light-emitting device. In the case of usingthe reflective films 2 as lower electrodes, a layer for increasing theefficiency of injection of carriers into the organic layer may also beprovided on the reflective films 2. For example, in the case of usingthe reflective films 2 as anodes, the efficiency of injection of holescan be improved by providing a layer of a material having a high workfunction. An electrically conductive metal oxide such as ITO or IZO canbe used as the material having a high work function. On the other hand,in the case of using the reflective films 2 as cathodes, the efficiencyof injection of electrons can be improved by providing a layer of amaterial having a low work function. An electron-injecting metalselected from alkali metals such as lithium, sodium and potassium,alkaline earth metals such as calcium, magnesium and strontium, andfluorides and so on thereof, or an alloy thereof with other metals or acompound thereof can be used as the material having a low work function.It is sufficient for the thickness of such a layer for increasing theefficiency of injection of carriers to be 10 nm or less.

The organic EL light-emitting layer 3 is formed on the reflective films2 formed as described above. In the organic EL light-emitting device ofthe present invention, the organic EL light-emitting layer 3 has astructure comprising at least an organic light-emitting layer, and ifnecessary a hole injection layer, a hole transport layer, and/or anelectron injection layer are interposed. Specifically, an organic ELlight-emitting layer 3 having a layer structure such as the following isadopted.

-   (1) Organic light-emitting layer-   (2) Hole injection layer/organic light-emitting layer-   (3) Organic light-emitting layer/electron injection layer-   (4) Hole injection layer/organic light-emitting layer/electron    injection layer-   (5) Hole injection layer/hole transport layer/organic light-emitting    layer/electron injection layer

Publicly known materials are used as the materials of theabove-mentioned layers. To obtain luminescence from blue to blue/greenin color, for example a fluorescent whitening agent of benzothiazoletype, benzimidazole type, benzoxazole type or the like, a metal chelatedoxonium compound, a styrylbenzene type compound, an aromaticdimethylidene type compound, or the like is preferably used in theorganic light-emitting layer. Moreover, a quinoline derivative (e.g. anorganometallic complex having 8-quinolinol as a ligand), an oxadiazolederivative, a perylene derivative, a pyridine derivative, a pyrimidinederivative, a quinoxaline derivative, a diphenylquinone derivative, anitro-substituted fluorene derivative or the like can be used for anelectron injection layer.

Next, the upper electrodes 4 are formed on the organic EL light-emittinglayer 3. In the case of using the reflective films 2 as anodes, theupper electrodes 4 will be cathodes, whereas in the case of using thereflective films 2 as cathodes, the upper electrodes 4 will be anodes.In the device of the present invention, light is extracted via the upperelectrodes 4, and hence the upper electrodes 4 must be transparent. Atransparent electrically conductive oxide such as ITO or IZO is thuspreferable as the upper electrodes 4 in the present invention.Furthermore, in the case of using the upper electrodes 4 as cathodes, alayer of a material having a low work function may be provided betweenthe transparent electrically conductive oxide and the organic ELlight-emitting layer 3, thus improving the efficiency of injection ofelectrons. As the material having a low work function in this case, anelectron-injecting metal selected from alkali metals such as lithium,sodium and potassium, alkaline earth metals such as, calcium, magnesiumand strontium, and fluorides and so on thereof, or an alloy thereof withother metals or a compound thereof can be used. To improve theefficiency of injection of electrons, it is sufficient for there to be alayer of the material having a low work function of thickness 10 nm orless, and moreover such a thickness is also preferable from theviewpoint of maintaining the required transparency.

The organic EL device of the present invention is preferably sealed toisolate the various constituent elements described above from thesurrounding environment. The sealing material is required to have lowpermeability to oxygen and moisture, high durability, and high heattransfer ability. Furthermore, in the case that the light from theorganic EL device is extracted via the sealing material, the sealingmaterial is required to be transparent to the light emitted by theorganic EL device. A commonly used material such as an acrylic resin canbe used as the sealing material.

With the organic EL device of the present invention, the light from theorganic EL light-emitting layer 3 may be used as is, or fluorescentcolor-converting material layers may be provided so that the light fromthe organic EL light-emitting layer 3 is subjected to wavelengthconversion. The fluorescent color-converting material layers may beformed on the upper electrodes 4, or may be formed on a separatetransparent substrate to form a color-converting filter, thecolor-converting filter then being bonded onto the EL device. Theformation of these layers and the bonding (including the formation oflayers required for the bonding) may be carried out using commonly usedmeans.

Moreover, the organic EL device of the present invention may emit onetype of light, or may emit a plurality of types of light of differentcolors. Preferably, the organic EL device is used as a display combinedwith a color-converting filter having red, green and blue light-emittingparts arranged in a matrix. In the case of using the organic EL deviceas a display, active matrix driving may be carried out using controllingelements such as TFTs, or passive matrix driving may be carried outusing upper and lower electrodes having line patterns that extend in twoorthogonal directions.

EXAMPLES 1 To 3

An Ni₃P film of thickness 100 nm was formed as a reflective film on aglass substrate by sputtering using a target having a composition ofNi₃P (Example 1). Moreover, for comparison, a sample in which, insteadof the Ni₃P, 100 nm of Al was formed by vapor deposition (Example 3),and a sample in which a metal electrode having a reflecting function wasnot formed, but rather 100 nm of amorphous In₂O₃:ZnO (ZnO molar ratio5%, hereinafter abbreviated to ‘IZO’) was formed (Example 2) weremanufactured.

Next, 10 nm of IZO was formed thereon by sputtering to match the workfunction to the injection level of the organic EL light-emitting layer.The electrode film was patterned using an ordinary photolithographyprocess using a mask giving a pattern of stripes with a width of 2 mmand a pitch of 0.5 mm, thus obtaining reflective films. These reflectivefilms were used as anodes. After that, the surface was subjected tocleaning using an oxygen plasma at room temperature.

The results of measuring the unevenness of the surface for the abovethree types of electrodes using an AFM are shown in Table 2. It can beseen that the surface roughness for the IZO/NiP/glass and the IZO/glasswas the same as that of glass within the scope of experimental error,but the unevenness for the IZO/Al/glass was more than 10 times greater.TABLE 2 Surface roughness of glass substrate and various types of lowerelectrodes Average surface Peak to Example roughness (nm) valley (nm) 1IZO (10 nm)/NiP 0.23 3.12 (100 nm)/glass 2 IZO (110 nm)/glass 0.23 3.153 IZO (10 nm)/Al (100 nm)/ 2.85 40.1 glass Glass 0.21 3.06

An organic EL light-emitting layer was formed on the anodes. Thestructure of the organic EL light-emitting layer was made to be a4-layer structure of hole injection layer/hole transportlayer/light-emitting layer/electron injection layer as an organic film;100 nm of copper phthalocyanine (CuPc) was formed as the hole injectionlayer, and 20 nm of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(α-NPD) was formed as thehole transport layer. Furthermore, 30 nm of4,4′-bis(2,2-diphenylvinyl)biphenyl(DPVBi) was formed as thelight-emitting layer, and 20 nm of aluminum tris(8-quinolinolate) (Alq)was formed as the electron injection layer. 20 nm of copperphthalocyanine (CuPc) was further formed to alleviate damage uponforming the upper transparent electrodes.

After the formation of the above films had been completed, 200 nm ofamorphous In₂O₃:ZnO (ZnO molar ratio 5%) was formed by sputtering usinga mask giving a pattern of stripes with a width of 2 mm and a pitch of0.5 mm extending in a direction orthogonal to the anode lines, thusforming cathodes. As a result, an organic EL light-emitting devicehaving a plurality of pixels of dimensions 2 mm×2 mm was obtained.

FIG. 2 shows the typical current-voltage characteristic for the threetypes of sample manufactured. The three types of sample showapproximately the same characteristic, this being because the layerstructure of the organic EL light-emitting layer, which determines thecurrent-voltage characteristic, is the same for each. Note, however,that for Example 3, there were parts (pixels) where an abnormal currentflowed due to short-circuiting occurring from the outset.

The light emission efficiency at a current density of 0.01 A/cm³ was 10cd/A for Example 1, 11 cd/A for Example 3, and 5 cd/A for Example 2. Theefficiency was low for Example 2 because there are no reflective filmsand hence light escapes from the rear surface, but taking this intoconsideration, it is thought that light-emitting devices ofapproximately the same characteristics were obtained for the threetypes.

The insulation breakdown voltage was measured as a quantitativeindicator of short-circuiting of pixels. In the case of TFT driving, avoltage is only applied in the forward direction, and hence essentiallyone should look at insulation breakdown by measuring a current thatflows excessively during forward bias, but there is a problem of the S/Nratio, and hence the excessive current cannot be detected. Here,evaluation was thus carried out by sweeping the voltage at 1 V/sec inthe reverse bias direction, and defining the voltage at which a currentof 1 μA was detected as the reverse bias breakdown strength.

FIG. 3 shows the frequency distribution of the reverse bias breakdownstrength obtained by carrying out measurements on 100 pixels for each ofthe organic EL light-emitting devices of Examples 1 to 3. It can clearlybe seen from FIG. 3 that the reverse bias breakdown strength is lowerfor the device of Example 3 (IZO/Al/glass) having severe surfaceunevenness. It is thought that this is because, due to the surfaceunevenness, there are parts where the thickness of the organiclight-emitting layer is low and hence the anode and cathode are close toone another. Note that out of the 11 pixels for which the reverse biasbreakdown strength was 0 to 9 V for Example 3, there was completeleakage for 5 of these pixels, with emission of light not being observedat all even upon applying a voltage in the forward direction.

Next, an investigation into minute defects in the pixels was carriedout. A current was passed in the forward direction, and the pixels wereobserved using a microscope so as to look for non-light-emitting pointsof size several μm to several tens of μm (dark spots). With the deviceof Example 3, approximately 20 dark spots were observed on average in a2 mm-square pixel, but with Examples 1 and 2, good devices wereobtained, with approximately 2 to 3 dark spots being observed at most ina pixel, and most of the pixels not having dark spots.

EXAMPLES 4 TO 8

Various reflective films were formed on glass substrates, varying thetarget, the film formation power, and the film formation time. Thethickness, reflectivity and surface unevenness for the reflective filmsobtained are shown in Table 3. TABLE 3 Film formation conditions andreflective film properties Surface unevenness Film Film Film AveragePeak to Target formation formation thickness Reflectivity roughnessvalley Example composition power (W) time (s) (nm) (%) (nm) (nm) 4 Ni₃P25 600 124 44.1 0.22 3.1 5 Ni₃B 111 212 139 39.4 0.21 3.13 6 Cr₃P 95 201124 56.6 0.23 3.15 7 Cr₃B 58 390 126 61.7 0.22 3.12 8 Cr₃B 25 393 51.462.3 0.2 3 Glass 0.21 3.06

From Table 3, it can be seen that the reflective films formed fromamorphous alloys have approximately the same flatness as the substratesurface.

The present invention can be used for an organic EL light-emittingdevice and a method of manufacturing the same. According to the presentinvention, as described above, the reflective films are formed using anamorphous alloy, whereby a good flat surface with reduced unevenness isobtained. With an organic EL light-emitting device in which an organicEL light-emitting layer and upper electrodes are formed on thereflective films, there is little occurrence of short-circuiting or darkspots, and hence an excellent device having good light emissionefficiency, high reliability, and improved image quality can beobtained.

1. An organic EL light-emitting device comprising: a substrate; thinfilms having a reflecting function formed on the substrate; an organicEL light-emitting layer; and upper electrodes; wherein the thin filmshaving a reflecting function are formed from an amorphous alloy.
 2. Theorganic EL light-emitting device according to claim 1, wherein theamorphous alloy is a metal-phosphorus alloy, the metal is one or aplurality selected from transition metals, and the metal-phosphorusalloy contains 10 to 50 at % of phosphorus.
 3. The organic ELlight-emitting device according to claim 1, wherein the amorphous alloyis a metal-boron alloy, the metal is one or a plurality selected fromtransition metals, and the metal-boron alloy contains 10 to 50 at % ofboron.
 4. The organic EL light-emitting device according to claim 1,wherein the amorphous alloy is a metal-lanthanide alloy, the metal isone or a plurality selected from transition metals, and themetal-lanthanide alloy contains 10 to 50 at % of a lanthanide.
 5. Theorganic EL light-emitting device according to claim 1, wherein an atomicradius ratio r/R (R>r) of elements constituting the amorphous alloy isnot more than 0.9.